// pku 2096
#include<stdio.h>
#include<string.h>

const int N=1004;

double E[N][N];
int n,s;


int main(){
	int i,j,k,t;
	scanf("%d%d",&n,&s);

	for(i=0;i<=n;i++){
        for(j=0;j<=s;j++){
            E[i][j]=0;
		}
	}
	for(i=n;i>-1;i--){
		for(j=s;j>-1;j--){
			if(i==n&&j==s) continue;	//e(n,s)=0
			E[i][j]=1;
			E[i][j]+=E[i+1][j]*(n-i)*j/n/s;
			E[i][j]+=E[i][j+1]*i*(s-j)/n/s;
			E[i][j]+=E[i+1][j+1]*(n-i)*(s-j)/n/s;
			E[i][j]/=(1.0-(double)(i*j)/n/s);

		}
	}

	printf("%.4f\n",E[0][0]);
	return 0;
}
